A gel consists of a cross-linked network of polymers swollen in a solvent. Under certain conditions, gels have been observed to undergo abrupt, reversible changes in volume. The volume change may be continuous or discontinuous. As an external environmental condition (e.g., temperature; solvent composition; pH; electric field; light intensity; pressure; ionic strength, etc.) is changed, the polymer network contracts and/or expands in volume (i.e., the gel is a "responsive" gel). The volume of such a gel may, under certain circumstances, change reversibly by a factor as large as several hundred when the gel is presented with a change in external conditions. See, Tanaka Phys. Rev. Lett. 40(12), 820-823 (1978); Tanaka et al, Phys. Rev. Lett. 38(14),771-774 (1977); Phys. Rev. Lett. 45(5),1636 (1980); Ilavsky, Macromolecules15, 782 (1982); Hrouz et al., Europ. Polym. J. 17, 361 (1981); Ohime et al, J. Chem. Phys. 8, 6379 (1984); Tanaka et al, Science 218, 462 (1982); Ilavsky et al, Polym. Bull. 7, 107 (1982); Gehrke, Responsive Gels: Volume Transitions II; Editor: K. Dusek Springer-Verlag New York, N.Y., pp. 81-144 (1993); Li et al, Annual Rev. Mater. Sci. 22, 243-77 (1992); and Galaev et al, Enzyme Microb. Technol. 15, 354-66 (1993), all of which are incorporated by reference. Because the volume change occurs in response to an external environmental stimulus, these gels are known as "responsive gels".
Responsive gels may be used as actuators suitable for application in servomechanisms and sensors, ranging from microscopic (silicon) mechanisms to larger devices comparable in size and force density to biological systems. For example, responsive gel actuators may be used as synthetic muscles which provide direct linear motion quietly, swiftly, and with useful force densities. Gels loaded with appropriate, beneficial solvents could be used in drug-release applications either in vivo or in vitro, releasing the solvent under appropriate environmental conditions. They could also be used to release or separate substances in a wide range of organic and inorganic chemical processes. Responsive gels have been used to encapsulate and release a variety of biologically active chemicals including pharmaceuticals such as aspirin, ibuprofin and enzymes. Gels could also be used to release or separate substances in a wide range of organic and inorganic chemical processes. The tremendous range of potential applications for gels is due in part to the large number of potential environmental conditions that can be used to induce a volume change in the gel.
In many of these applications, it is difficult to directly introduce the environmental change necessary to induce (or "trigger") the desired volume change in the gel. For example, it may be difficult to directly heat in the vicinity of a synthetic muscle deeply imbedded in a servomechanism or a drug delivery device located in vivo. A method for remotely activating a gel phase transition is desirable under these circumstances. Preferably, such a remote triggering method is non-intrusive. In addition, the method of remotely activating a gel phase transition should provide for the local, and not systemic, administration of the environmental "trigger".
Although electromagnetic radiation has been used in connection with animal or human tissue, it has the disadvantage that it can interact strongly with the tissue to produce undesirable effects. Examples range from sunburn caused by ultraviolet light or bulk heating and cooking caused by microwave radiation.
Animal tissue is relatively transparent to a static or quasi-static electric or magnetic field. Remote heating of animal or human tissue has been accomplished by the implantation of particulate magnetic material into the animal tissue, followed by application of a magnetic field to the area. In this case, the magnetic material may couple with the magnetic field to generate heat through hysteresis and eddy current. However, there has been no application of magnetically induced remote heating to gels or to hydrous materials other than animal tissue.
It is the object of the present invention to provide a system and method for remotely triggering gels to undergo a phase transition. It is a further object of the present invention to induce a phase transition in a gel by application of a magnetic, electric or electromagnetic field. It is yet a further object of the present invention to provide a system for remotely triggered delivery of drugs.